时间转换（Date、Calendar）以及时间戳等

1、将字符串转化为date类型、将时间戳转化为date类型

    @Test
public void show(){
        long timestamp = 1621234567890L; // 假设有一个时间戳
// 创建一个SimpleDateFormat对象，指定日期时间的格式
        SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
// 将时间戳转换为Date对象
        Date date = new Date(timestamp);
// 使用SimpleDateFormat格式化Date对象为字符串
        String formattedDate = sdf.format(date);
// 输出结果
        System.out.println("时间戳：" + timestamp);
        System.out.println("转换后的日期：" + formattedDate);
}

2、将字符串转化为时间戳

@Test
public void get(){
    String timeString = "2021-01-01 00:00:00";
    String pattern = "yyyy-MM-dd HH:mm:ss";
    SimpleDateFormat sdf = new SimpleDateFormat(pattern);
    try {
        Date date = sdf.parse(timeString);
        long timestamp = date.getTime();
        System.out.println("时间戳：" + timestamp);
    } catch (ParseException e) {
        e.printStackTrace();
    }
}

3、使用Calendar日历对象完成

//            为日历对象定义一个指定日期的数据
    @Test
public void User(){
        Calendar st=Calendar.getInstance();
          st.set(2009,6 - 1,12);

        System.out.println("年"+st.get(Calendar.YEAR));
        System.out.println("月"+st.get(Calendar.MONTH));
        System.out.println("日"+st.get(Calendar.DATE));

4将日历对象转化为时间戳(两种方法)

 // 创建Calendar对象
    Calendar calendar = Calendar.getInstance();
    // 将Calendar对象转换为时间戳
    long timeStamp = calendar.getTimeInMillis();
    // 输出时间戳
    System.out.println("时间戳：" + timeStamp);

//第二种

  // 转换为时间戳
    Date date = calendar.getTime();
    long timestamp = date.getTime();

    // 输出结果
    System.out.println("Date: " + date);
    System.out.println("Timestamp: " + timestamp);

5、两个时间类型的比较

  @Test
public void Ust(){
        Calendar calendar1 = Calendar.getInstance();
        Calendar calendar2 = Calendar.getInstance();

        calendar1.set(2020, 1, 1);
        calendar2.set(2021, 1, 1);

        if(calendar1.before(calendar2)){
            System.out.println("calendar1在calendar2之前");
        }else if(calendar1.after(calendar2)){
            System.out.println("calendar1在calendar2之后");
        }else{
            System.out.println("calendar1和calendar2相同");
        }

}

6、LocalDateTime时间类型演示

// 获取当前时间
// 根据指定的日期创建对象
// 往日期中进行加减法

  public void Loca(){

        LocalDateTime localDateTime=LocalDateTime.of(2018,1,12,4,1,1);
        System.out.println(localDateTime);
        LocalDateTime localDateTime1=LocalDateTime.now();
        LocalDateTime plusyearsResult=localDateTime.plusYears(2L);
        LocalDateTime plusMonths=localDateTime.plusMonths(3L);
        LocalDateTime plusDays=localDateTime.plusDays(7L);
        LocalDateTime plusHours=localDateTime.plusHours(2L);
        LocalDateTime plusMinutes=localDateTime1.plusMinutes(10L);
        LocalDateTime plus=localDateTime.plusSeconds(10L);
        System.out.println("当前时间是："+localDateTime+"\n"+
                "当前时间加2年后为："+plusyearsResult+"\n"+
                "当前时间加3个月后："+plusMonths+"\n"+"当前时间加七日后为："
                +plusDays+"\n"+"当前时间加2小时后："+plusHours+"\n"+"当前时间加十分钟后为"+
                plusMinutes+"\n"+"当前时间加十秒后为"+plus);

    }

//          获取日期的年月日周时分秒
    public void Dome(){
        LocalDateTime localDateTime=LocalDateTime.now();
        int year=localDateTime.getDayOfYear();
        int MONth=localDateTime.getDayOfMonth();
        DayOfWeek dayOfWeek=localDateTime.getDayOfWeek();
        System.out.println("今天是："+localDateTime+"\n"+
                "本年当中第"+year+"\n"+"本月当中第"+ MONth+"\n本周当中第"+dayOfWeek.getValue()+
                "-即"+dayOfWeek+"\n");

        int years = localDateTime.getYear();
        Month month = localDateTime.getMonth();
        int day = localDateTime.getDayOfMonth();
        int hour = localDateTime.getHour();
        int minute = localDateTime.getMinute();
        int second = localDateTime.getSecond();
        System.out.println("今天是"+ localDateTime + "\n"
                +"年:" +years + "\n"
                +"月︰" +month.getValue()+"-即"+month +
                "\n"+"日" +day + "\n"
                +"时:" +hour + "\n"+"分:" + minute + "\n"+"" +
                "秒:" + second + "\n"
        );

    }

//    比较两个日期
//    转时间戳，考虑时区问题
    @Test
    public void Dome1(){
        //判断两个时间点的前后
        LocalDate localDate1 = LocalDate.of( 2017,8,8);
        LocalDate localDate2 = LocalDate.of( 2018,8,8);
        boolean date1IsBeforeDate2 = localDate1.isBefore(localDate2);
        System.out.println("date1IsBeforeDate2 : " + date1IsBeforeDate2);
        // date1IsBeforeDate2 == true
        
        Instant instant = Instant.now();
        //2019-06-08T16:50: 19.174z
        System.out.println(instant);
        Date date = Date.from(instant);
        Instant instant2 = date.toInstant();
        //Sun Jun 09 00: 50:19 CST 2019
        System.out.println(date);
//2019-06-08T16:50: 19.174z
 System.out.println(instant2);

    }

7 计算两个日期间的间隔

    @Test
public void Dome2(){
    System.out.println("-------------------------------");
    LocalDateTime date3 = LocalDateTime.now();
    LocalDateTime date4 = LocalDateTime.of(2018, 1, 13, 22, 30, 10);
    Duration duration = Duration.between(date3, date4);
    System.out.println(date3 + " 与 " + date4 + " 间隔  " + "\n"
            + " 天 :" + duration.toDays() + "\n"
            + " 时 :" + duration.toHours() + "\n"
            + " 分 :" + duration.toMinutes() + "\n"
            + " 毫秒 :" + duration.toMillis() + "\n"
            + " 纳秒 :" + duration.toNanos() + "\n"
    );

8. 自定义格式输出,日期类型转化为字符串

  LocalDateTime date1 = LocalDateTime. now();
    DateTimeFormatter formatter2 = DateTimeFormatter.ofPattern("yyyy年MM月dd日 HH:mm:ss");
            String date2Str = formatter2.format(date1);
    System.out.println(date2Str);
    //            将字符串转化为日期
}

   @Test
public void Dome3(){
    String datetime = "2018-01-13 21:27:30";
    DateTimeFormatter dtf = DateTimeFormatter.ofPattern("yyyy-MM-dd HH:mm:ss");
    LocalDateTime ldt = LocalDateTime.parse(datetime,dtf);
    System.out.println(ldt);
//    毫秒转日期？
        System.out.println("---------long豪秒值转换为日期---------");
        DateTimeFormatter df= DateTimeFormatter.ofPattern("yyyy-MW-dd HH:mm:ss");
        String longToDateTime = df.format(LocalDateTime.ofInstant(
                Instant.ofEpochMilli(System.currentTimeMillis()),ZoneId.of("Asia/Shanghai")));
        System.out.println(longToDateTime);
    }